ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 403
Note E. Art. 171.
If we suppose the angle of incidence exactly equal to 45°, assume $ for the refractive index of the fluid, and apply Fresnel's formulae to calculate the ratio of the intensity of light reflected at the exterior surface of a bubble, and polarized in a plane perpendicular to the plane of incidence, to that of light similarly reflected and polarized in that plane, we find 0*228 to 1, a ratio which certainly differs much from one of equality. But in order to render the two intensities equal, it is sufficient to increase the angle of incidence by only 3° 35'; and in fact, as a matter of convenience, the position of the observer was usually such that the deviation, of the light was somewhat greater than 90°, and therefore the angle of incidence somewhat greater than 45°.
Note F. Art, 191.
I have since received a slab of glass of the kind here recommended, which has been executed for me by Mr Darker 3f Lambeth, and which answers its purpose admirably, the medium being eminently sensitive. Besides its general use as ^ screen, this slab, from its size and form, has enabled me to brace further than I had hitherto done (Arts. 75, 76) the connexion between certain fluctuations of transparency which }hc medium exhibits and corresponding fluctuations of sensibility.
Note G. Art. 192.
Paper washed with a mere infusion of the bark of the horse-chestnut is quickly discoloured; but a piece washed with a solution which had been purified by chemical means remained white, and proved exceedingly sensitive.
Note H. Art. 204
I have since ordered a complete train of quartz, of which a considerable portion, comprising among other things two very fine prisms, has been already executed for me by Mr Darker. With these I have seen the fixed lines to a distance beyond // more